The present application relates to blending of powders and, more particularly, relates to the blending of moisture-sensitive powders using liquid nitrogen or liquid alcohols for use in making thermal battery separators.
Battery separators are physical barriers interposed between the anode and the cathode in a thermal battery that prevent physical contact therebetween. The separator provides ionic conductivity during discharge and is generally in the form of a pressed powder pellet commonly comprising a binder and a eutectic mixture (electrolyte) of lithium chloride and potassium chloride or of lithium fluoride, lithium chloride and lithium bromide, although other mixtures are known. The electrolyte is generally incorporated into an inert binder such as powdered magnesium oxide, zirconia, or aluminum nitride, which immobilize it by capillary action when the electrolyte is molten. The separators must be stable in the cell environment, resisting degradation by the cell media, must permit conduction across the separator of current transferring ions or charges and must be capable of operation under the conditions of use.
Formation of the separator from the powdered or particulate binder and electrolyte requires blending or mixing of the materials. It is difficult to obtain homogeneous mixtures of powders or finely divided solids by conventional dry mixing techniques, especially where there are significant differences in particle size, particle shape and specific gravity. Therefore, the blending for separator production has typically been accomplished using chlorinated or fluorinated hydrocarbon liquids such as the group of compounds under the name Freon™, marketed by E. I. DuPont. However, Freon™ can no longer be used because of environmental constraints and prohibitions of the use of chlorinated and fluorinated hydrocarbons.
Often, reactive liquids are used in the blending of solids where the blending occurs before substantial reaction of the liquid occurs, where the reactive liquid later reacts to form a solid matrix. However, if reaction of the blending liquid with the powders is not desired, liquids are required that are nonreactive, that have properties that facilitate the blending of the powders and that sometimes can also be removed from the blended powders. In blending of some solid particles, such as propellants or explosives, gaseous nitrogen has been adsorbed onto particles surfaces which thereby maintain the particles in a non-agglomerating, free-flowing state to allow subsequent mixing to be accomplished by simple tumbling. In other cases, liquid nitrogen is used to make the solids more brittle so that the solids can be ground and then mixed.
Useful would be a method for mixing of moisture-sensitive powders using inexpensive and non-reactive liquids.